Lactic acid is widely used in food and chemical raw materials used for medicine, cosmetics, etc. Further, polylactic acid obtained by using lactic acid has attracted attention as a biodegradable plastic which is degraded finally to carbon dioxide and water by microorganisms. Therefore, it is necessary to produce lactic acid at a low cost with a high productivity.
As a method for producing lactic acid, a biological production method in which a saccharide is fermented by lactic acid bacteria. However, since the acid resistance of lactic acid is low, in order to achieve a high productivity of this method, the lactic acid produced by fermentation is required to be converted into a lactic acid salt by neutralizing it with an alkali. Such neutralization with an alkali requires a step for restoring lactic acid from the lactic acid salt, whereby the production steps become complicated and the production cost becomes high.
As a method for obtaining lactic acid without carrying out neutralization with an alkali, a method which uses a transformant prepared by introducing a gene encoding LDH into a yeast host. For example, Patent Document 1 discloses that lactic acid can be produced with a high productivity, without carrying out a neutralization step with an alkali, by cultivating a transformant containing an LDH gene which is derived from mammals such as human and is introduced into a S. pombe host, wherein a gene that is a part of a group of genes encoding pyruvate decarboxylase of the S. pombe host is deleted or inactivated. Further, Patent Document 2 discloses that lactic acid can be obtained by using a transformant prepared by introducing an L-lactate dehydrogenase gene of Lactobacillus plantarum into a Saccharomyces cerevisiae that does not essentially produce ethanol when it is cultured in a culture medium.
Further, Patent Document 3 discloses that a transformant containing an LDH gene of Lactobacillus pentosus (LpLDH gene) introduced into a S. pombe host, wherein a gene that is a part of a group of genes encoding pyruvate decarboxylase of the S. pombe host is deleted or inactivated, has a lactic acid production capability at the same level as or a greater level than the transformant described in Patent Document 1. And particularly, it discloses that, by introducing an LpLDH gene in combination with a LDH gene derived from human (HsLDH gene), the lactic acid production capability can be improved significantly as compared with a transformant transformed with a single copy of HsLDH gene or a transformant transformed with a single copy of HsLDH gene and a single copy of a LDH gene derived from other species.